High-performance carbon fibers are classified as either PAN-based carbon fibers obtained using polyacrylonitrile (PAN) as the starting material, or pitch-based carbon fibers obtained using a series of pitches as starting material.
Carbon fibers are widely used for space and aviation, construction and civil engineering, and sports and leisure purposes, taking advantage of their extremely high strength and elastic modulus compared to ordinary synthetic polymers.
With recent increasing interest in methods for more efficient use of energy to achieve energy savings, Joule heat generation by accelerated CPUs and electronic circuits is becoming a significant issue. Overcoming such problems requires “thermal management”, to handle the heat in an efficient manner.
Carbon fibers have high thermal conductivity compared to ordinary synthetic polymers, and still further increase in heat conduction is being investigated. However, the thermal conductivity of most currently marketed PAN-based carbon fibers is lower than 200 W/(m·K), and they are not necessarily satisfactory from the viewpoint of thermal management. Pitch-based carbon fibers, on the other hand, have a high degree of graphitization and are therefore recognized as being more capable of achieving high thermal conductivity than PAN-based carbon fibers.
Commonly known heat-conductive fillers include those filled with metal oxides, metal nitrides, metal carbides and metal hydroxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, aluminum hydroxide and the like. However, metal material-based fillers have high specific gravity, and their weights are therefore increased when used to form composite materials. Also, addition of spherical materials such as carbon black as a carbon-based material in large amounts causes “powder falling”, while the conductivity adversely affects devices, particularly electronic devices. Carbon fibers, on the other hand, have low specific gravity and therefore offer the advantage not only of lowering the weight of the composite material when added to the same volume as a metal material-based filler, but also of preventing powder falling due to their fibrous form.
Incidentally, such composite materials are sometimes used to connect heating units and heat sinks. When a highly rigid resin composition is used in such cases, gaps are often created between the heating unit and heat sink, making it impossible to achieve efficient heat conduction. Composite materials with greater flexibility and greater ability to follow the shapes of heating units and heat sinks have therefore been desired.
The features of composite materials used for thermal management will now be described. For efficient utilization of the high thermal conductivity of carbon fibers, it is preferred for the carbon fibers to form a network within some sort of matrix. Especially when the network is formed in a three-dimensional manner, the high heat conduction of the carbon fibers is exhibited not only in the in-plane direction of the molded article but also in its thickness direction, which is considered to be highly effective for radiator plate uses, for example. However, while fibers used in the past as woven fabrics to form composite materials in combination with matrices have improved thermal conductivity within the plane, they exhibit less than satisfactory heat conduction in the thickness direction due to the inability to adequately form a carbon fiber network.
In view of this situation, many attempts have been made to more radically improve the thermal conductivity of carbon fibers. For example, Patent document 1 discloses a thermal conductive molded article with high mechanical strength obtained by impregnating carbon fibers which are aligned in a single direction, with graphite powder and a thermosetting resin. Also, Patent document 2 discloses improvement in the physical properties such as thermal conductivity by enhancing the physical properties of the carbon fibers, but it is unclear whether definite improvement is achieved in the thermal properties of molded articles.    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 5-17593    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 2-242919